1. Field of the Invention
The present invention relates to the treatment of ores containing leachable metal values. More specifically, this invention relates to the recovery of copper values from copper ores, and is particularly applicable to leaching of secondary copper sulfides from any copper sulfide deposit and extraction of copper therefrom. In a preferred embodiment, the invention relates to a hydrometallurgical treatment of sulfide minerals found in porphyry ore deposits, which are generally difficult to leach in an efficient and economical manner.
2. Description of the Prior Art
In treating copper bearing ores, materials containing primary or secondary sulfides have typically been processed using the conventional milling/flotation process which includes crushing, grinding and flotation, followed by smelting and refining of the concentrate. Copper oxide minerals are not easily floated and these ores have generally been processed hydrometallurgically by sulfuric acid leaching in slurry, vat or heap leaching processes. In recent years, bio-heap hydrometallurgical processing of secondary sulfides ores using a ferric sulfate lixiviant has gained some favor. Research has also been intense in recent years on leaching of copper sulfide concentrates, including chalcopyrite concentrates, using slurry bio-leaching, atmospheric leaching of ultra-fine ground concentrates and processes involving pressure leaching at elevated temperatures.
Conventional milling/flotation typically requires a particle size reduction to less than 150 mesh (0.105 millimeters) to achieve mineral liberation from the gangue and to permit high rougher flotation copper recovery. Regrinding of the rougher concentrate produced to as fine as minus 400 mesh (0.037 millimeters) may then be necessary to allow mineral liberation sufficient to achieve an economic concentrate grade. The concentrate produced must then be further processed by smelting and refining or a hydrometallurgical process to finally obtain cathode copper. The conventional milling/flotation process is mineralogy dependent, and is energy, capital and operating cost intensive, as are the subsequent smelting and refining steps, requiring a higher ore grade to justify project economics.
Hydrometallurgical leaching processes for ores of copper oxides, secondary sulfides and dump leaching of chalcopyrite waste materials have typically included in situ leaching, dump leaching, heap leaching, vat leaching and agitation slurry leaching.
In situ leaching processes involve in-place leaching of material in a deposit. Porosity for solution permeability is required either naturally, by high pressure, hyro-fracturing, or by blasting. Rubble zones or caved areas from old underground mines are also suitable for in-situ or in-place leaching. Solution is distributed on the surface or injected through drill holes in the deposit, percolates or is forced by pressure through the ore zone, solubilizing the metal values. The leach solutions are collected by underground workings or drill wells for recovery of the metal values therefrom. Typically, recovery of metal values requires years and only reaches leach recoveries of 40 to 60 percent.
Dump leaching is typically applied to leaching of the massive run of mine tonnages of mineralized, but below ore cutoff grade waste, generated from copper porphyry deposit mine operations. Dump leaching can be used to recover copper from materials containing oxide and sulfide mineralization and utilizes bio-leaching techniques. Recoveries are typically less than 50 percent after 10 to 25 years of leaching. Leach kinetics are very slow and solution copper contents are very low.
Heap leaching is applied for leaching of oxidized copper ores, secondary copper sulfides, uranium and precious metal ores. Typically the ores are crushed to less than one inch (25 millimeters) and to as fine as minus 1/4 inch (6.3 millimeters). Leaching can be performed in permanent heaps where successive lifts are placed over the original lift or in a reusable pad which allows the ore to be leached in one lift, the leach residue and a new lift placed on the pad. Recoveries are generally 65 to 85 percent, depending or the ore being leached, and leach cycle times range from months to a year. Heap leaching has been used more recently for bio-leaching of secondary copper sulfide ores such as with the operations of Quebrada Blanca and Cerro Colorado in Chile and Cerro Verde in Peru. Cyprus Miami in Arizona, USA, and other US producers also employ a ferric leach or ferric cure technology for run of mine mixed oxide and secondary sulfide copper ores. Heap leaching technology allows processing of higher grade ores, but is typically used on lower grade ores (less than 1% copper) due to comparatively low capital and operating costs versus conventional technology.
Heap leaching of secondary copper sulfides of chalcocite and covellite is a viable but challenging hydrometallurgical process. The dedicated secondary sulfide leaching facilities started up over the past few years have generally experienced lower recoveries, slower leach kinetics and higher operating costs than predicted from test work. The major difficulty has been oxygen availability internally within the heap sufficient to promote bacterial activity for direct leaching of sulfides and/or the oxidation of ferrous sulfate to ferric sulfate, the primary lixiviant for the process. Many techniques from fine crushing to drum agglomeration, various lift heights, various flow rates and flow regimes and forced aeration have been employed to enhance the process. The end result remains that this technology has distinct limitations and disadvantages.
Slurry agitated leaching has been used primarily on oxidized copper ores. It can also be used for bio-leaching of copper sulfide concentrates. Slurry leaching requires fine grinding and continuous agitation which results in high power consumption and is typically applied to higher grade ores or concentrates.
Vat leaching has typically been used for processing of copper oxide ores and those ores with higher copper grades.
Although the present invention is equally effective on any leachable copper containing ore, it is particularly effective on leaching of the lower grade secondary sulfide copper ores commonly found in porphyry copper deposits.
It is well known that the bulk of the world copper resources are contained in porphyry copper deposits. Porphyry deposits originate as intrusions of protore, generally with chalcopyrite mineralization. When rock porosity is present to allow downward flow of meteoric water, and provided sufficient pyrite is present to produce oxidizing acids, surface minerals are dissolved and transported downward to areas where solutions become more basic and reducing, generally below the water table, and are reprecipitated. Thus, there are typically three copper mineralized zones in classical porphyry deposit, the oxidized zone, the supergene zone (which is generally the highest grade zone in the deposit that contains secondary sulfide minerals) and the hypogene zone or protore zone which is presumably the original source of all the copper in the deposit.
Thus, a classical description of a porphyry copper deposit includes a relatively copper barren oxidized capping over the deposit; lying beneath this capping is a zone of oxidized copper mineralization (oxide copper ore); beneath this zone is a zone of enriched secondary sulfides; and beneath the secondary sulfides zone lies the zone of primary sulfides or protore from which the deposit was generated.
Porphyry deposits can deviate from the classical model due to age of the deposit, mineralization, ground water table variations over time, erosion, climatic conditions, etc. Thus, deposits can have little to no oxide capping or a large oxidized capping zone, and little or no secondary sulfide enrichment or large enrichment zones. The majority of the in ground copper resources worldwide are contained in primary ore zones as chalcopyrite (70%). However, substantial resources are also contained in secondary sulfide mineral zones and in oxide zones. For example, in Morenci, Ariz. or in El Salvador, Chile, the economic value of such deposits is due to supergene enrichment. The lower grade secondary sulfide ores generally found in porphyry copper deposits range from 0.5% to 1.5% copper contained in chalcocite and covellite mineralization.
Oxide ores are generally contained in the rock as fracture filled mineralization and most copper values are contained as chrysocolla, malachite, azurite and atacamite. There are numerous other minor oxide minerals present in such deposits. Oxide minerals are normally easily leachable simply by leaching with sulphuric acid. Leach kinetic rates are rapid leading to effective recoveries.
Secondary sulfide ores are generally contained as fracture filled mineralization in the rock fractures, but usually include some dissemination of the minerals in the host rock. The bulk of secondary sulfide minerals occur as chalcocite, but can also have significant quantities of covellite, some bornite and some deposits can contain quantities of enargite. There are also minor amounts of other sulfide minerals.
Secondary sulfide minerals are not efficiently leached with sulfuric acid, but can be leached efficiently under oxidizing conditions using lixiviants such as ferric sulfates or chlorides. Chalcocite (Cu.sub.2 S) leaching with ferric sulfate proceeds in two stages. The first stage leaching involves dissolution of one of the copper molecules from the Cu.sub.2 S, leaving CuS or what is termed synthetic covellite. The first stage leaching is relatively temperature insensitive and has very rapid kinetics. The second stage leaching of the synthetic covellite solubilizes the remaining copper, leaving the sulfur in elemental sulfur form. This stage of leaching can have an order of magnitude slower leaching kinetics, is very temperature dependent and is also dependent on ferric concentration. However, leaching rates are still commercially acceptable since the synthetic covellite resulting from the chalcocite leaching has a relatively porous structure. Naturally occurring covellite, not uncommon in porphyry deposits, is significantly more difficult to leach than synthetic covellite. Bornite minerals are leachable in ferric sulfate with kinetics similar to synthetic covellite, while enargite can be slightly to totally refractory to leaching.
Primary sulfides are generally contained within fracture filled mineralization, but increasing amounts are disseminated within the host rock. The mineralization is mostly chalcopyrite. Leaching of the primary chalcopyrite is especially difficult in both ferric sulfate and chloride at ambient temperature and pressure. Leachability increases with increasing temperature, stronger oxidants, and pressure above atmospheric. At the present time, there is no commercial heap leaching process for chalcopyrite ore that provides a mine with a leach scenario such as with secondary sulfides. Leach kinetics are several orders of magnitude slower than with secondary sulfides.
Typically, with all mineral leaching, recovery and leach kinetics increase with decreasing particle size. Thus, for example, in U.S. Pat. No. 4,115,221 there is described a process for ferric sulfate leaching of copper sulfide-bearing materials which requires fine-grinding such materials to a particle size of at most one micron, prior to leaching. In U.S. Pat. No. 5,917,116 the copper mineral is milled to the particle size P-80 of between 2 and 20 microns and then subjected to oxidative leaching in the presence of chloride ions. In Canadian Patent No. 1,156,050 a process is disclosed for recovering copper from chalcopyrite in which the material is first ground to a particle size of 1.5-5 microns and then divided into two streams each of which is subjected to a particular hydrometallurgical treatment. Also, in published Canadian Patent Application No. 2,215,963 an atmospheric mineral leaching process is disclosed in which a sulfide mineral composition is first milled to a particle size P-80 of 20 microns or less before leaching with a solution comprising sulfuric acid and ferric ions.
It is also known to extract copper values from ores by dump leaching or pile leaching where the dump or pile of the ore is wetted with sulfuric acid to sulfatize the same and extract copper. An example of such dump leaching is disclosed in U.S. Pat. No. 4,120,935 which has one of the inventors common to one of the inventors in the present application. A similar pile leaching is disclosed in U.S. Pat. No. 5,527,382 where the ore is classified into coarse and fine fractions, with the coarse fraction being subjected to pile leaching and the fine fraction to pile curing followed by repulping under pressure, filtering and washing. Then, the raffinate from both fractions is subjected to solvent extraction and electrowinning. Despite the fact that such processes have achieved some saving in the time period required to leach the dump or pile, the sulfatization still requires several days or even weeks to achieve satisfactory recoveries and, in fact, the use of sulfuric acid alone may not be at all satisfactory for treatment of secondary sulfide minerals containing chalcocite and covellite.